Typically, tires are manufactured from a carcass having a substantially U-shaped section with metal beads at the inner perimeters of the walls. Support can be provided to a tire carcass by steel cord belt plies extending around the outer periphery of the carcass and across the width of the tread. Such steel belt plies are relatively inextensible in the intended direction of rotation of the tire and include steel belt cords disposed parallel to the direction of rotation or at a low angle thereto. The belt plies act to restrict the movement of the tread and give the tire better road-holding properties. Support such as this type is particularly important in a radial tire, where the carcass is formed from a rubberized woven fabric having relatively inextensible reinforcing cords running transversely, i.e. radially, from bead to bead.
To further improve the high speed performance, some tires have one or more cap plies laid circumferentially over the belt plies. A cap ply is a layer comprising cords (typically nylon or polyester) encapsulated in a rubber composition that is laid over all or a portion of the axial width of the steel belt plies. These cap plies are not found in all tires; they are mostly used on tires with higher speed ratings to help all the components of the tire stay in place at high speeds.
The rubber composition used in the cap ply must have good adhesion to the cords used in the cap ply, good hysteresis so that heat generation is low, and good mechanical properties (e.g., modulus and elongation).
The inventors have discovered a rubber composition suitable for use in the cap ply of a tire that possesses a good balance of the above properties.